The invention relates to an apparatus and a method of reducing turbulent drag on an object moving relative to a fluid.
Several approaches have been proposed to reduce drag caused by turbulence and boundary layer flow around an object moving relative to a fluid. Drag can be reduced by appropriately shaping the object to minimize the formation of vortices, for example, by installing the so-called riblets. Drag may also be reduced by actively controlling vortex formation in the boundary layer by applying electromagnetic fields, by heating the surface, by generating micro-bubbles, by suctioning off the inner boundary layer and/or by injecting a high-velocity flow into the boundary layer. Some of the active control measures require the fluid to be electrically conducting, such as an electrolyte, sea water and an ionized gas, whereas other active control measures require complex control mechanisms and/or may use more energy in operation than is being saved through the drag reduction.
It would therefore be desirable to provide a more energy-efficient method for reducing the drag on an object moving through a fluid, and an apparatus using the method.
According to one aspect of the invention, drag can be reduced by producing along a major surface of the object a traveling wave force traveling in the span-wise direction, i.e., in a direction parallel to the major surface and perpendicular to the stream-wise (flow) direction. This approach does not require a closed loop control mechanism and is therefore robust and cost-effective.
According to another aspect of the invention, the force may be produced by an arrangement of magnets having poles with identical polarity in the span-wise direction and poles of alternating polarity in the stream-wise direction, as well as electrodes producing an electric field substantially perpendicular to the flow direction. The electrode spacing can be selected essentially independent of the optimum wavelength of the traveling wave for reducing the turbulence by applying a suitable pulse sequence to the electrodes.
According to yet another aspect of the invention, the force may be produced by an array of actuators, for example, mechanically deformable tiles disposed on the surface. The tiles may be formed of a resilient material or of a so-called xe2x80x9csmartxe2x80x9d material which has a shape memory effect and does not demand a continuous activation, thereby conserving energy.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments and from the claims.